Preheat your oven to 350°F and lightly grease a 9x13 inch baking dish with butter or cooking spray. This ensures your dump cake won't stick and will bake evenly. Having the dish ready before you start layering saves time and keeps your ingredients organized.

Spread the trimmed rhubarb pieces evenly across the bottom of your prepared baking dish. Sprinkle the strawberry gelatin powder and sugar directly over the rhubarb, distributing them as evenly as possible. The gelatin and sugar will create a syrupy base as the rhubarb releases its juice during baking, so don't worry about mixing—the heat and moisture will combine everything naturally.

Sprinkle the yellow cake mix evenly over the rhubarb-gelatin mixture, covering it completely. I like to use my fingers to distribute the mix so it settles into an even layer—this helps the cake bake more uniformly. Don't stir or mix; the dry cake mix will absorb the moisture from below and create a tender crumb as it bakes.

In a small bowl, whisk together the melted and cooled butter, cold water, vanilla extract, and salt until combined. Slowly pour this mixture evenly over the entire surface of the dry cake mix. I recommend pouring in a slow, steady stream to help the liquid distribute evenly rather than pooling in one spot—this ensures the whole cake bakes consistently and rises properly.

Place the dish in the preheated 350°F oven and bake for 45 to 60 minutes, until the top is golden brown and the cake is set but still has a slight jiggle in the center when gently shaken. The exact time depends on your oven, so start checking around 45 minutes. The rhubarb will have released its juices and created a lovely sauce on the bottom while the cake rises and browns on top.

Remove the dump cake from the oven and let it cool at room temperature for at least 15-20 minutes before serving—this allows the structure to set slightly and makes it easier to portion. Serve warm or at room temperature with a dollop of whipped cream or a scoop of vanilla ice cream to complement the tart rhubarb and sweet cake.